Mass spectrometer tube



Feb. 1, 1966 REICH 3,233,098

MASS SPECTROMETER TUBE Filed June 15, 1962 2 Sheets-Sheet l BATTERY IONCURRENT MEASURING DEVICE ow SUPPLY Fig. I

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GUNTER RE/CH ,4 TTOR/VE'V Feb. 1, 1966 G. REICH MASS SPECTROMETER TUBE 2Sheets-Sheet 2 Filed June 15, 1962 ION T CURRENT MEASURING DEVICE POWERSUPPLY BATTERY Fig. 2

v Mm w m n 7 J A United States Patent 4 Claims. oi. 250-419 Theinvention deals with a mass spectrometer tube for the detection of ionsof various masses, especially for partial pressure measuring devices inthe high-vacuum range, whereby the ions resulting from electronsproduced from a cathode in an ion source are directed in the form of astream of ions, through a sorting space in which a high frequencyalternating field of variable frequency is applied to sortingelectrodes, so that only ions having a mass corresponding to a givenapplied frequency can reach the target electrode, the current of whichserves as a measure of the proportion of this specific ionic mass in thegiven mixture.

When a mass spectrometer of the above described type is used, then aresidual current is superimposed on the current produced at the targetelectrode by the ions of a given mass. This residual current is presenteven in the case of a frequency far removed from the resonant conditionof the ionic masses and is dependent both on pressure and frequency andcan also have both positive and negative signs. This residual currentdiminishes the detecting efficiency, in respect to components of lowintensity and can also, especially at higher pressures, assume suchlarge values that a perfect analysis of the separate components will notbe possible. Each decrease of the residual current will result,therefore, in an increase of the detecting efiiciency as well as awidening of the range of application of such measuring devices in thefield of higher pressures.

Certain devices for the decrease of the residual current have beensuggested already, which include the high frequency sorting system, thatis, sorting electrodes positioned between grid electrodes with negativepotential. In addition, solid apertured shield electrodes for screeningof the ion source from the sorting system and which permit the stream ofions-to pass through, have been utilized. The mechanical screeningbetween the ion source and the sorting system results in an additionaldisadvantage, since the areas enclosing the ion source and the sortingsystem must be evacuated separately. Such a device is especiallydisadvantageous when mass spectrometers are used as partial pressuremeasuring devices in high vacuum technique. Furthermore, devices builtaccording to this principle cannot be designed as built-in measuringunits, in which all of the electrodes are supported by one and the sameflange, making it possible to install the measuring system directly inthe vacuum container.

Tests carried out concerning the cause of the disturbing residualcurrent have shown the following:

(1) Non-resonant charged particles may leave the sorting system sidewiseby means of a velocity component in a direction which deviates from thatof the stream of ions and find electric field forces which acceleratethem in the direction of the target electrode.

(2) Resonant and non-resonant particles create secondary particles whenhitting the surface of the sorting system. These secondary particlessuch as, for example, secondary electrons can have velocity componentsdeviating from the direction of the beam of ions and similarly reach thetarget electrode other than by way of the sorting system.

(3) The ion source does not provide a concentrated "ice beam of ionswith velocity components of all ions in parallel directions. Due to thefinite generating angle, the ions may appear in the zone enclosing thesorting system and from there reach the target electrode without passingthrough these sorting systems.

The present invention intends to create a mass spectrometer tube which,by means of a special arrangement and design of screening elements, willallow only such ions as have passed through the sorting system reach thetarget electrode, while stray particles will be intercepted by thescreening elements to a substantial extent and therefore will notinfluence the target electrode current. According to this invention thisis achieved in such a way that tubular screening elements are providedfor in the areas of both the ion source and the target electrode,whereby the axis of symmetry of these elements is parallel to the streamof ions, and that these tubular screening elements are arranged inrelation both to the ion source and the target electrode in such a waythat the particles can neither leave the ion source with a. velocitycomponent which deviates to any considerable degree from the center lineof the stream of ions nor reach the target electrode from the insidespace of the tube. Different designs of such a total screening arepossible as to their details. In an advantageous type of design onescreening element, as an anode of the ion source, is shaped by twocoaxial cylindrical pieces of different diameters, whereby thecylindrical piece with the smaller diameter is placed on the side of thesorting space and is equipped at both end faces with electricallyconduct-ive grid electrodes. These grid electrodes may have a metallicconnection with the cylindrical piece.

Another advantageous design provides for a hairpinshaped cathode for theion source, whereby this hairpin cathode penetrates with its point anopening of a screening electrode, which in turn is placed inside thatcy1indri cal piece of the anode which has the larger diameter. Thepotential applied to the screening electrode which adjoins the cathodeis preferably negative in relation to the cathode potential.

In connection with the screening element constructed as the anode of theion source and consisting of two cylindrical pieces, the cathode canalso be enclosed by a further tubular screening electrode which issimilarly equipped at its end adjacent the sorting space with a gridelectrode. The cathode, the grid electrode and the further tubularscreening electrode preferably operate at the same potential as that ofthe cathode.

In addition it may be of advantage to equip the target electrode with atubular screening part which extends up to the range of the sortingspace and which at least covers the last sorting grid of the sortingspace. In a modified design the entire sorting space or a substantialpart of it is encompassed by this screen. It might be expedient thereby,in the case of a device which has, in an otherwise known manner, both aretarding grid and an electrically biased screening grid, to provide anapertured disc electrode, preferably of the same potential as that ofthe tubular screening element, between this electrically biasedscreening grid and the target electrode.

The measure described will achieve a very considerable decrease of theresidual current, while the detecting efficiency will be correspondinglyincreased. Apart from the separate useful designs of the screeningelements it appears to be fundamentally essential to apply a totalscreening, that is, both as far as the ion source and the targetelectrode is concerned. Test results have shown that neither thescreening of the ion source nor that of the target electrode in itselfalone makes it possible to suppress satisfactorily the residual current,even in the case of a complicated geometrical and electrical design.

The drawing shows the diagrammatical view of forms of construction of amass spectrometer tube in accordance with the invention, as follows:

FIGURE 1 a mass spectrometer tube with cathode cylinder,

FIGURE 2 a mass spectrometer tube with hairpin cathode.

Both figures show the cross-section of a cylindrical mass spectrometertube 1, which can be connected by means of a connecting piece to avacuum device not represented on the drawing.

Tube 1 of FIGURE 1 is equipped with a coiled heated cathode 310 and ananode 410 of an ion source, screening grids 5 and 6, a retarding grid 7,a target electrode 811) and a sorting space consisting of sortingelectrodes 9 through 19. The heated cathode 310 is enclosed by a cathodecylinder 311 which has the same potential as that of the cathode and thesurface of which is shaped to form the grid 312, screening the cathodetowards the side of the connecting piece 2.

The anode 410 of the ion source consists of two concentric tubularcylinders which have different diameters 411 and 412 and which areconnected with each other by means of a circular disc 413, whereby thecylinder with the larger diameter 411 partially overlaps the cathodecylinder 311 and also contains on its inside an entrance grid electrode414. An exit grid 415 is placed on the tubular cylinder 412 on that sideof the anode 411 which faces the sorting space.

The ion catcher electrode 81% is entirely surroundedwith the exceptionof the recess $11-by a screening 812 which has likewise the samepotential as that of the cathode. The screening in the shape of acylinder 813 may stretch along the entire length of the sorting space orat least overlap the sorting electrode 19, as can be seen on diagram 2.

FIGURE 2 shows inside the tube a hairpin cathode 320 which penetrateswith its point an opening in the screening electrode 321. An anode 420which corresponds to anode 4-11! in FIGURE 1 consists in a similar wayof tube-shaped cylindrical pieces 421 and 422 with an inserted circulardisc 423, whereby the cylinder 421 surrounds the screening electrode321. Grid electrodes 424 and 425 are provided on the cylindrical part422 in the area of the end faces. The screening 813 of the targetelectrode 310 in the form of construction as shown in FIGURE 2 reachesonly as far as the last sorting electrode 19 of the sorting space.

The heated cathodes 310 and 320 (FIGURES 1 and 2, respectively) areheated by means of a filament battery 20. The high frequency current forthe grids 919 of the sorting space is provided by a tunablehigh-frequency generator 21. A one-side grounded measuring device 22shows the ion current reaching the target electrode 810. All of thedirect-current voltage potential as well as the comparison voltage forthe control of the high frequency amplitude are supplied by a commonpower-supply unit 23.

The mass spectrometer tubes, in accordance with the invention, do notdiifer from the known constructions as far as their basic operation isconcerned. The ions are drawn out of the ion source through thescreening grid 5 which is negatively biased to a high degree, wherebythe cylinders 412 and 422 (FIGURES 1 and 2, respectively,) serve toinsure an exit which is nearly parallel to the center line of the tube,so that a well focused beam Will enter the sorting space,

By means of a special design of ion sources, such as is made possible byelectron-optical systems, a well focused stream of ions is directed intothe sorting space, whereby the special forms of construction of thescreening at the ion collector electrode 810 provide protection againstthe stray particles. The positively biased retarding grid 7 can bepenetrated only by such particles as fulfill the resonant conditions inrespect to the high frequency voltage in the sorting space and thus haveabsorbed sufficient energy. The negatively charged screening grid 6suppresses disturbances which might appear due to secondary emissions ofthe surfaces within the range of the ion collector electrode 810.

The novel form of construction thus achieves a considerable increase ofthe detecting efiiciency and can be utilized in the field of higherpressures.

I claim:

1. A mass spectrometer tube of the type in which ions resulting from theelectron beam of a cathode in an ion source are directed in the form ofa beam of ions through a sorting space, in which a high frequencyalternating field of variable frequency is present on a plurality ofsorting electrodes so that only ions having a mass corresponding to agiven frequency can reach a target electrode, the current of whichserves as a measurement of the partial pressure of this given ionic massin the given mixture; the improvement comprising a cylindrical anodeelement positioned adjacent the ion source and having a central axisparallel to the directed ion beam, a cylindrical exit element of smallerdiameter than and concentric with said cylindrical anode element, saidcylindrical exit element extending beyond said cylindrical anode elementin the direction of the target electrode, an entrance grid electrodepositioned at the end of said cylindrical anode element facing the ionsource, an exit grid electrode positioned at the end of said cylindricalexit element facing the target electrode, a conductive tubular screeningpart enclosing the target electrode and extending into the sorting spaceso as to overlap at least one sorting grid therein, and means forestablishing the same potential on said cylindrical anode element as onsaid cylindrical exit element. 7

2. A mass spectrometer tube according to claim 1 wherein the ion sourcecomprises a hairpin-shaped cathode which extends into said cylindricalanode element.

3. A mass spectrometer tube according to claim 1 wherein the ion sourceis located within a cathode cylinder, and which has one end closed by anelectrically conductive grid electrode and an open end facing saidcylindrical anode element.

4. A mass spectrometer tube according to claim 1 including a retardinggrid and an electrically biased screening grid, an apertured discelectrode positioned between said electrically biased screening grid andthe target elecrode, and means for establishing said apertured discelectrode at the same potential is that of the tubular screeningelement.

References Cited by the Examiner UNITED STATES PATENTS 2,758,214 8/1956Glenn 250-41.9 2,818,507 12/1957 Britten 250-41.9 2,847,574 8/1958Donner 25041.9 X 2,975,277 3/1961 Von Ardenne 2504l.9

RALPH G. NELSON, Primary Examiner.

1. A MASS SPECTROMETER TUBE OF THE TYPE IN WHICH IONS RESULTING FROM THEELECTRON BEAM OF A CATHODE IN AN ION SOURCE ARE DIRECTED IN THE FORM OFA BEAM OF IONS THROUGH A SORTING SPACE, IN WHICH A HIGH FREQUENCYALTERNATING FIELD OF VARIABLE FREQUENCY IS PRESENT ON A PLURALITY OFSORTING ELECTRODES SO THAT ONLY IONS HAVING A MASS CORRESPONDING TO AGIVEN FREQUENCY CAN REACH A TARGET ELECTRODE, THE CURRENT OF WHICHSERVES AS A MEASUREMENT OF THE PARTIAL PRESSURE OF THIS GIVEN IONIC MASSIN THE GIVEN MIXTURE; THE IMPROVEMENT COMPRISING A CYLINDRICAL ANODEELEMENT POSITIONED ADJACENT THE ION SOURCE AND HAVING A CENTRAL AXISPARALLEL TO THE DIRECTED ION BEAM, A CYLINDRICAL EXIT ELEMENT OF SMALLERDIAMETER THAN AND CONCENTRIC WITH SAID CYLINDRICAL ANODE ELEMENT, SAIDCYLINDRICAL EXIT ELEMENT EXTENDING BEYOND SAID CYLINDRICAL ANODE ELEMENTIN THE DIRECTION OF THE TARGET ELECTRODE, AN ENTRANCE GRID ELECTRODEPOSITIONED AT THE END OF SAID CYLINDRICAL ANODE ELEMENT FACING THE IONSOURCE, AN EXIT GRID ELECTRODE POSITIONED AT THE END OF SAID CYLINDRICALEXIT ELEMENT FACING THE TARGET ELECTRODE, A CONDUCTIVE TUBULAR SCREENINGPART ENCLOSING THE TARGET ELECTRODE AND EXTENDING INTO THE SORTING SPACESO AS TO OVERLAP AT LEAST ONE SORTING GRID THEREIN, AND MEANS FORESTABLISHING THE SAME POTENTIAL ON SAID CYLINDRICAL ANODE ELEMENT AS ONSAID CYLINDRICAL EXIT ELEMENT.